x²+y²/x－y=（x+y）(x-y)/(x-y)=x+y>=2根号xy=2
楼主再算算
(x^2+y^2)/(x+y)=[(x-y)^2+2xy]/(x-y)=(x-y)+2/(x-y)≥2根号（2）当且仅当x-y=2/(x-y)时，取等号！
